Open it up and count the number of wires there are in the ribbon cable that connects the CD board to the mainboard.
If there are 20, you can forget about easily getting a mod. 20 pin mods are quite rare, but they do occaisionallly turn up.
If there are 21, you then need to look at the CD board. Have a look and see if it has a rectangular 32 pin IC, a square 64 pin IC or no obvious chips on it. If it has no ICs and a 'PC Trap' board in the middle of the ribbon cable, it seem that you're pretty much out of luck. If there isn't a PC trap, it can be modded.
If it has the 32 pin IC, that's very easily modded and you can use a 21 pin mod from Lik-Sang or Lan Kwei.
If it has the 64 pin IC, that's also easily modded, but it seems you need to buy a mod specifically from Gamegizmo. Don't ask me why, as I don't know, but those ones work. They are a little more expensive than Lik-Sang's or Lan-Kwei's mods.
Can't help you with the number of different US models. Maybe there's an FAQ over at Gamefaqs that might help.